1. Field of the Invention
This invention relates to a process for producing "high-quality" carbonaceous pitch suitable for use in a variety of chemical and physical situations such as in the production of carbon electrodes for aluminum electrolysis cells, steel arc furnaces, or other electrochemical processes, or as an industrial binder for use in roofing, road construction materials and similar applications. Some available sources of tar or pitch, for example mild coal gasification processes or sub-standard coke oven tars, do not produce pitch having the specifications required by such applications. The process of this invention not only produces pitch suitable for use in such applications, converting low-quality pitches to high-quality pitches, but also increases the pitch yield over known processes without sacrificing pitch quality. In particular, low-quality pitches are improved by flash thermocracking, yielding 25 to 26 wt % high-quality pitch compared to the process of this invention by which pitch yield is increased to about 28 to 40 wt % without sacrificing quality.
2. Description of Prior Art
Flash thermocracking of low-temperature coal tars to produce a binder pitch is known, having been studied by the U.S. Bureau of Mines in the period 1963-1973. See Berber, J. S. et al, "Low-Temperature Lignite Tar: Processing and Utilization" Bulletin 663 United States Department of the Interior, Bureau of Mines, 1973. U.S. Pat. No. 5,091,072 teaches a continuous process for preparing high softening point pitches comprising heat treating a heavy oil or pitch by dispersing the heavy oil or pitch in a gas stream of an inert gas or superheated vapor, as fine oil droplets, and bringing the dispersed fine oil droplets into contact with the inert gas or superheated vapor, at a temperature of 350.degree. to 550.degree. C. under a reduced or normal pressure. In accordance with the teachings of the '072 patent, the temperature range of 350.degree. to 550.degree. C. is indicated to be critical because, if the temperature is not high enough, removal of the light fractions is insufficient whereas, if the temperature is too high, excessive thermal polymerization such as coking tends to take place, even though the time required for the treatment is short. Dispersion of the preheated heavy oil or pitch in accordance with the teachings of the '072 patent are carried out by application of a centrifugal force to the preheated heavy oil or pitch by a rotating structure such as a disk, a cone or a bowl rotating at a rate so as to introduce the droplets so formed into the gas streams substantially perpendicular to the direction of the gas flow. The pitch produced in accordance with the process disclosed by the '072 patent has quinoline insolubles (QI) of less than 1 wt %.
U.S. Pat. No. 3,928,170 teaches a process for manufacturing a highly aromatic pitch by heat treating an aliphatic hydrocarbon-containing petroleum-based residual oil for efficient cracking, poly-condensing and aromatizing thereof. The heat treatment is conducted in such a manner that the raw material oil is brought into direct contact with a non-oxidizing gas or a perfectly combusted gas containing substantially no oxygen, as a heat carrier gas, heated to 400.degree. to 2000.degree. C. To effect thermocracking, aromatization and poly-condensation, the temperature of the raw material oil is maintained within the range of 350.degree. to 450.degree. C. Contact of the raw material oil with the heat carrier gas is achieved by blowing the heat carrier gas into the raw material oil or, alternatively, employing a jet scrubber. The residence time of the raw material oil within the reactor through which the heat carrier gas is blown is in the range of 0.5 minutes up to 20 hours, depending on the preheat temperature of the raw material oil.
Thermocracking of a raw material oil to produce pitch is taught by U.S. Pat. No. 4,554,148 in which the raw material oil is subjected to thermocracking conditions at a temperature between 400.degree. to 500.degree. C. while removing cracked, light hydrocarbon components to obtain a pitch product containing mesophase and light hydrocarbon components. U.S. Pat. No. 4,127,473 teaches a method for the batch thermocracking of heavy oils, such as steam blowing for production of binder pitch, employing a reactor having a rotary injection pipe which is rotatable within the reactor. Upon completion of the thermocracking and withdrawal of the reaction product, the injection pipe, while in rotation, ejects preheated raw material under pressure against the interior wall surfaces of the reactor to remove coke which is deposited on the reactor walls during the previous cracking operation.
U.S. Pat. No. 5,198,101 teaches a process for producing an anisotropic pitch in which a metal alkylaryl sulfonate is combined with a carbonaceous feedstock substantially free of mesophase pitch and heated for a period of time at an elevated temperature while passing a non-oxidative sparging gas such as nitrogen through the feedstock. Similarly, U.S. Pat. No. 4,999,099 teaches a process for producing an anisotropic pitch in which a carbonaceous feedstock alone is heated at elevated temperature while passing a reactant sparging gas therethrough. U.S. Pat. No. 4,758,329 teaches a coking process in which the coke co-efficient of thermal expansion (CTE) is reduced and coke particle size is increased by sparging with a gas during the coking cycle.
See also Stadelhofer, J. W. et al, "The Manufacture of High-Value Carbon from Coal-Tar Pitch", Fuel., 60:9, 877-882 (1981) which teaches delayed coking and horizontal chamber coking for the production of cokes with low sulfur and metal content and the manufacture of hard pitch by means of a continuous flash process with optimized thermal and pressure treatment of pitch to facilitate the "tailored" manufacture of binder pitches of different qualities.
It is an object of this invention to provide a process for producing high-quality pitches suitable for use as binders for carbon electrodes or other graphitized articles, or as a binder for roads, roofs, and other industrial and commercial markets.
It is another object of this invention to provide a process for producing high-quality pitches from low-quality pitches whereby the yield of pitch is increased without sacrificing pitch quality.
These and other objects of this invention are achieved by a process for producing high-quality pitches in which preheated coal tar and/or preheated low quality pitch s atomized, forming an aerosol. The aerosol is injected into a reaction vessel in which it is contacted with a flowing, non-reactive gas atmosphere for less than about 10 seconds. The temperature of the gas atmosphere is in the range of about 1400.degree. F. to about 2000.degree. F. A range of gaseous, liquid and solid products are formed in the reaction vessel. The fraction of the liquid product that fails to distill at temperatures below about 750.degree. F. is separated from the distilled product and recovered as product pitch. This combination of flash thermocracking and atomization of the preheated feed to the reaction vessel results in higher yields of product pitch and lower yields of product coke than flash thermocracking without atomization.